The present invention relates to a combustion-type power tool having a combustion chamber frame, a push lever and a connection unit.
A conventional combustion-type driving tool such as a nail gun is disclosed in U.S. Pat. No. 5,197,646. In the disclosed driving tool, a spring serving as a biasing member is positioned at a lower portion of the tool for urging a push lever downwardly toward a workpiece. A spring seat protrudes from an outer peripheral surface of a cylinder, so that the spring is seated on the spring seat and interposed between the spring seat and a housing serving as an outer frame of the nail gun. The spring urges an arm portion provided integrally with the push lever for biasing the push lever downwardly.
In the above-described conventional combustion type power tool, since the spring is interposed between the outer peripheral surface of the cylinder and the housing for urging the push lever downwardly, a surplus space is required inside the housing for accommodation of the spring in the housing to inadvertently increase an outer diameter of the housing. The diameter of the housing can be reduced by reducing the diameter of the cylinder. However, the diameter of the piston reciprocally moved in the cylinder must also be reduced, if the diameter of the cylinder is reduced. Then, pressure applied to the piston is reduced due to the small diameter of the piston, which in turn, lowers output to lower the driving power.
Still however, the diameter of the housing must be as small as possible in case of a driving work at a specific working environment such as immediately beside a wall, even though the diameter of the housing cannot be set too small.
Further, in the combustion type power tool, the cylinder etc., are heated by the combustion heat because driving power is generated as a result of combustion of a fuel, and the exhaust gas is discharged through an exhaust hole formed in the cylinder. Since the arm portion is positioned nearby the cylinder, the arm portion is exposed to heat due to the heated cylinder, and the arm portion has high temperature. Moreover, since the exhaust hole is positioned near a connecting position between the push lever and the arm portion, the arm portion is exposed to the exhaust gas having a high temperature. In this connection, the arm portion must be made from a heat resistant material in addition to the general requirement of a mechanical strength. Stainless steel is used for the arm portion as a material for fulfilling these requirements.
Even though stainless steel has the above described performance, stainless steel is relatively expensive as a material cost. FIG. 6 shows a configuration of the arm portion which is made by punching a stainless steel plate, and FIG. 7 shows a punching arrangement for the arm portions. Since the arm portion is produced by bending an integral plate member formed by punching, residual surplus regions are provided in the steel plate after punching, which degrades yieldability.
Further, since the heated portion of the arm portion is exposed to an atmosphere, a user may burn himself if he may touch the heated portion.